Export of STL file enlarged very slightly for the diameter
In the modeling tab and object mode, pressing n gives me the dimensions of the object and this is at odds with the location of the widest points on the x and y axes, which can be found by switching to edit mode and selecting the appropriate vertex.
There seems to be a mismatch between the 2.
Location  x = 10.098  times 2 gives me roughly 20.2 which is what I want.
Dimension x = 21.5 which is definitely not right and it seems to be this dimension which is output to the stl file.
What I did to fix this: switch to object mode, select the object, press S to scale and then scaled by 0.939873418 for the X and Y axis and 1.0 for the Z axis and this corrected the dimension error. Inspecting the location of the widest point on the x axis shows that it has not moved.
Export of the STL shows the object now has the correct dimensions :-)
I've no idea what caused the problem in the first place.